Natural gas is often available in areas remote to where it will be ultimately used. Quite often the source of this fuel is separated from the point of use by a large body of water and it may then prove necessary to transport the natural gas by large vessels designed for such transport. Natural gas is normally transported overseas as cold liquid in carrier vessels. At the receiving terminal, this cold liquid, which in conventional practice is at near atmospheric pressure and at a temperature of about -160.degree. C. (-256.degree. F.), must be regasified and fed to a distribution system at ambient temperature and at a suitable elevated pressure, generally around 80 atmospheres. This requires the addition of a substantial amount of heat and a process for handling LNG vapors produced during the unloading process. These vapors are sometimes referred to as boil-off gases.
Many different processes have been proposed for handling boil-off gases produced during LNG unloading. The amount of boil-off gases can be significant, particularly if the LNG is unloaded at higher pressures. In some LNG unloading processes, the vapor left in the storage container can constitute up to about 25% of the product mass, depending on the LNG pressure and composition. One option for recovering the boil-off vapor is to pump it out of the storage container for use as a natural gas product. The horsepower required to run evacuation pumps increases and is an added expense to the overall expense of a LNG unloading process. The industry has a continuing interest in processes that minimize the horsepower requirements of making the boil-off vapors available for commercial use.
Many suggestions have also been made and some installations have been built to use the large cold potential of the LNG. Some of these processes use the LNG vaporization process to produce by-product power as a way of using the available LNG cold. The available cold is used by using as a hot sink energy sources such as seawater, ambient air, low-pressure steam and flue gas. The heat-transfer between the sinks is effected by using a single component or multi-component heat-transfer medium as the heat exchange medium. For example, U.S. Pat. No. 4,320,303 uses propane as a heat-transfer medium in a closed loop process to generate electricity.
The LNG liquid is vaporized by liquefying propane, the liquid propane is then vaporized by seawater, and the vaporized propane is used to power a turbine which drives an electric power generator. The vaporized propane discharged from the turbine then warms the LNG, causing the LNG to vaporize and the propane to liquefy.
Although the use of LNG as a cold sink is known in the art, there is a continuing need for an improved process that uses the cold sink of the liquefied natural gas and at the same time economically and efficiently processes boil-off gases from liquid natural gas for use as a product.